“The portrait of Tutu is a national icon in Nigeria, and of huge cultural significance,” said Giles Peppiatt, Bonham’s director of modern african art.

Peppiatt uncovered the work after a family in north London contacted him following recent lucrative sales of Nigerian artworks at auction. He said the family had been “pretty astounded” to learn it was “a missing masterpiece”. “It is very exciting to have played a part in the discovery and sale of this remarkable work,” he said.

Woman in the picture: is Nigeria’s legendary Tutu still alive?

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The Booker prize-winning novelist Ben Okri told AFP this month that the painting had taken on almost mythical status in his native Nigeria, where it was thought of as the African Mona Lisa. “It has been a legendary painting for 40 years. Everybody keeps talking about Tutu, saying: where is Tutu?” he said after a viewing at Bonhams. “He wasn’t just painting the girl, he was painting the whole tradition. It’s a symbol of hope and regeneration to Nigeria, it’s a symbol of the phoenix rising.”

Enwonwu, who died in 1994, is considered the father of Nigerian modernism. He made three paintings of Tutu, the locations of all of which had been a mystery until the recent discovery. The works became symbols of peace after the clash of ethnic groups in the Nigerian-Biafran conflict of the late 1960s.

Enwonwu’s work Negritude, also painted in the 1970s, fetched £100,000 in the same sale.